The present invention relates to improvements in fluid power systems having multiple, separately controllable double-acting fluid motors, and more particularly to such systems used for powering multiple mechanical functions of lift truck load handling attachments.
Double-acting fluid motors, which as used herein broadly include such devices as bidirectional cylinder and piston assemblies and bidirectional rotary fluid motors, conventionally require a pair of hydraulic or pneumatic lines connecting each motor with its respective control valve so that the control valve may direct pressurized fluid respectively to one or the other of the lines to actuate the motor in either of two directions, the line not receiving pressurized fluid serving to exhaust fluid from the motor. When multiple separately controllable double-acting fluid motors are utilized in a fluid power system, each performing a separate function, each motor conventionally has its own pair of fluid lines interconnecting it with its own separate control valve. Thus the number of fluid lines interconnecting the control valves with such separately controllable double-acting motors is normally twice the number of motors. Such separately controllable motors are to be distinguished from motors which operate in tandem in a cooperative manner to perform a single function, such as a pair of double-acting hydraulic cylinders which extend or retract in unison to raise or lower a boom, or the like. Such tandem motors are not separately controllable since they do not perform separate functions, and therefore may be connected in parallel through only one pair of fluid lines to a single control valve.
The multiplicity of fluid lines normally required by multiple, separately controllable double-acting fluid motors causes substantial problems in certain fluid power applications, notably where the multiple motors are remote from the control valves and more through extended distances with respect to the valves. For example in an industrial lift truck the control valves would normally be mounted on the main body of the truck within reach of the operator, while the various separately controllable double-acting motors might be mounted on a load handling attachment which reciprocates vertically through a substantial distance on the lift truck mast. Load handling clamp attachments for lift trucks, for example, have two or three separately controllable double-acting hydraulic motors performing such diverse functions as clamping, rotating and side-shifting of the load. Under conventional practice, three such separately controllable fluid motors would require six fluid lines extending from three valves on the lift truck body to the vertically reciprocating attachment. Because of the substantial movement of the attachment with respect to the valves, such lines would have to be extensible and retractable, requiring hose reels capable of handling six lines. While hose reels capable of serving such a large number of lines might be used, the bulk and space requirements of both the reels and the lines are unacceptable in light of the limited space available for such items on the mast of a lift truck, and the impingement of such items upon the visibility of the lift truck operator.
One solution to this problem, limited only to the specific application of a particular type of clamp having two double-acting fluid motors, has been proposed in Lake U.S. Pat. No. 3,692,198 wherein a means of serving the two motors with less than the normal number of extensible and retractable fluid lines between the motors and the control valves is disclosed. However Lake's arrangement of control valves in parallel with one another requires three three-position control valves to operate the various functions and is adaptable to control only two separately controllable double-acting hydraulic motors. Both a simplification in the control valve arrangement and, more important, an adaptability to control more than two separately functioning double-acting hydraulic motors, is required for certain important types of lift truck load handling attachments.
An alternative solution to the foregoing problem of multiplicity of fluid lines has been known for many years. In this alternative, which is of more generalized application than Lake's specific two-motor application, the problem is solved not by reducing the number of lines between motors and control valves but rather by mounting some of the control valves so as to move in unison with the motors, such as upon the vertically reciprocating lift truck load handling attachment. Control valves which move in unison with a load handling attachment can all be jointly connected to the main body of a lift truck by merely a single pair of extensible and retractable fluid lines for which hose reels are required. However such control valves must be controlled electrically by solenoid operation because of their remote location with respect to the operator. This electrical control requires an extensible and retractable multi-conduit electrical cable extending between the lift truck body and the load handling attachment. While the electrical cable is much more compact than the equivalent number of hydraulic lines which would otherwise be required, the constant flexing of the metal strands in the electrical cable in response to the movement of the load handling attachment causes work-hardening and resultant breakage and short-circuiting of the cable, resulting in troublesome, recurrent service problems.
Accordingly an alternative solution which does not require extensible and retractable electrical lines and cables, which has a more simplified control valve arrangement, and which is adaptable to any number of separately controllable double-acting hydraulic motors, is required.